The present invention relates generally to implantable medical devices, and more particularly to an implantable device for detecting and monitoring the progression of congestive heart failure.
Many patients who have suffered one or more myocardial infarctions subsequently require treatment for congestive heart failure (CHF). The left heart fails while the pumping function of the right heart remains adequate, because the latter has only about 20% of the workload of the former. This leads to an increase in blood volume congested to the lungs, resulting in pulmonary congestion, build up of edema, and congestion of internal organs including the stomach and intestines. Increased fluid in the stomach and intestines reduce their ability to absorb drugs prescribed for treatment of CHF, particularly diuretics. The congestion is often accompanied by a worsening of myocardial function, with consequent drop in blood pressure and reduced renal perfusion, which only further aggravates the congestive situation. Thus, late recognition of congestion leads to increased dosages of oral diuretics that are unsuccessful to treat the condition, ultimately requiring that the patient be hospitalized.
Avoidance of hospitalization and the pitfalls of late treatment require detection of CHF at an early stage, so that the prescribed drugs can be fully absorbed and effective. If detected early, a combination of diuretics and other drugs can slow the progress of the disease and allow the patient to enjoy an improved lifestyle.
The implantable medical device of the '771 application is of a size smaller than a typical pacemaker device—about the size of a thumb. It is implanted in a subcutaneous pocket in the patient's chest, under local anesthesia and minimally invasive requirements. The device includes a hermetically sealed can with appropriate electronic circuitry inside. A set of can-mounted electrodes is used to measure the impedance of the adjacent tissue and most especially the lung tissue. The progressive retention of fluid in the lungs and congestion of the ventricle together result in a reduced impedance measurement that is monitored either continuously or periodically by the device.
In a preferred mode of operation, the device disclosed in the '771 application alerts the patient and the attending physician when a diagnostic threshold is reached which is indicative of the progression of CHF. The overall architecture of the device follows implantables practice, and is a stand-alone monitoring device. However, the partitioning of the device is flexible and the division of sensing and analysis structures can be shared between implanted and external (remote, i.e., non-implanted) devices. Conventional programming and telemetry links can be used to connect the implanted device to the remote device. For example, the signal processing may be performed entirely internally within the device, or the device may operate as a data logger and communicate with an external programmer device which participates in data reduction and analysis.
It is a principal aim of the present invention to provide an implantable heart failure monitor and ventilation measuring implant that constitutes an improvement over the device of the '771 application, capable of better quantifying and detecting the status of the patient, and especially of achieving very early detection of CHF.
Past attempts made to quantify and detect the cardio-pulmonary status of the patient include the following. U.S. Pat. No. 5,003,976 to Alt discloses a technique to access the cardio-pulmonary status of the patient more accurately from a single sensor located within the heart. U.S. Pat. No. 5,331,966 to Bennett et al. discloses a subcutaneous implant that measures the electrical cardiac signal detected in the body upon depolarization of the heart, using electrical contacts. Tachycardia from the far field electrocardiogram can be recorded to measure capture detection, and a threshold adaptation can be used to save energy when the device is implanted or to provide timing windows for anti-tachycardia pacing therapy, and a control signal for use in adjusting pacing rate to physiologic demand. These techniques utilize intrinsic signals that derive from the patient's own heart beat.
U.S. Pat. No. 5,876,353 to Riff discloses a method to measure edema by evaluating the patient's respiration rate. According to the patent, the respiratory rate of the patient can be detected from a subcutaneous location, and increases with increasing congestion. A subcutaneous implant used for that purpose has an electrode that connects the implant to location within the heart. However, the patent does not indicate a technique to measure ventilation, which is the product of tidal volume and respiratory rate.
U.S. Pat. No. 5,957,861 to Combs et al discloses a monitor together with a pacemaker to detect the occurrence of edema by monitoring a DC (direct current, or base) value. According to the patent, respiratory events are noise that should be filtered out. The disclosed device measures impedance both locally, from electrodes on the device, and from electrodes implanted in the patient's heart. Local impedance measurement is described in the patent as a local edema indicator, not as representing lung edema, but rather with an assumption that lung edema is represented by a transthoracic measurement. The local edema is compared to changes in the transthoracic value to reflect pulmonary, edema. The determinations are based upon comparison of the short term and long term averages, and the discrimination from local edema compares the changes in the local pocket edema to the changes in the transthoracic edema. Signals produced by activity or breathing are excluded from the analysis so to find the DC signal level; all other values are deemed to be noise components.
U.S. Pat. No. 5,987,352 to Klein et al. discloses a minimally invasive implant and a preferred method for detecting physiologic events, by storing the intrinsic heartbeat of the patient—the EKG (electrocardiogram) signals. The EKG signal and additional sensor data may be communicated through telemetry such as used for pacemakers and other implant devices. An elliptical or rectangular shape of less than ½-inch diameter is disclosed.
U.S. Pat. No. 6,190,324 to Klieval, et al. discloses an implantable medical device for tracking patient cardiac status. This disclosure, which is incorporated herein by reference in its entirety, compares heart rate and activity level of the patient, detected from a mechanical electrical converter such as an accelerometer, to derive a quotient of heart rate and activity that is indicative of the patient's cardio-pulmonary status. Other rate signals, such as respiratory rate, may be used as an alternative to the patient's activity level to develop the heart rate activity coefficient. Such an implant is projected to be used in patients with a normal heart rate response, in which a comparison of information from heart rate to the amount of activity would allow a determination of the patient's cardio-pulmonary status.
U.S. Pat. No. 6,512,949 to Combs et al discloses an impedance monitor that detects by DC measurements the amount of edema and additionally works in conjunction with a loop recorder, pacemaker, or defibrillator. The patent further discloses that respiration methods signals are considered to be noise, and are filtered out to obtain the DC value.